PROJECT SUMMARY Autism spectrum disorder (ASD) affects at least 1 in 59 children in the U.S. and places an enormous economic burden on our health care system. A core behavioral feature of ASD is disruption of social interactions. Recently, we have focused study on the pathology of the ventral anterior cingulate cortex, specifically Brodmann area 24 (BA24), in ASD. This is a brain area that plays a prominent role in mediating social behavior. We discovered a deficit in the expression of NTRK2 in pyramidal neurons of layer III of BA24 in ASD brain donors relative to typically developing (TD) control donors. NTRK2 is the gene that encodes the TrkB receptor and mediates BDNF signaling in these neurons. BDNF/TrkB signaling is involved in numerous neuronal functions during development and in adulthood, including neuronal differentiation, synaptic plasticity, dendritic spine formation and maintenance, and glutamatergic and GABAergic synapse development. Here, we hypothesize that a core neuropathology of ASD is a disturbance in BDNF-TrkB signaling in BA24 neurons of the anterior cingulate cortex that results in subsequent alterations of glutamatergic and GABAergic neurons that disrupts the balance of excitatory and inhibitory influences in this region and is associated with increased expression of neuroinflammatory mediators in the same brain region. In addition to addressing this hypothesis using ASD and control human brain tissues, the proposed animal studies are a critical step to determining how well these animal models recapitulate the molecular pathology of human ASD. Major objectives of Aim 1 of this study are to examine gene and protein expression levels of major synaptic proteins in laser-captured excitatory pyramidal neurons and immunologically-defined inhibitory GABAergic neurons from postmortem BA24 from ASD and matched TD donors, using psychiatrically characterized ASD and control brain tissues from national brain repositories. Additionally, Aim 1 will determine neuroinflammatory mediator gene and protein expression levels of laser captured layers II-III of the ACC in the same donor population. The objective of Aim 2 is to examine the same signaling and synaptic proteins in excitatory pyramidal and inhibitory GABAergic neurons laser captured from the anterior cingulate cortex of BTBRT+Itpr3tf/J (BTBR) mice and mice prenatally exposed to valproic acid and poly I:C (with corresponding controls) at two developmental stages (P21 and P120). Aim 3 will measure dendritic and soma elements of pyramidal neurons in two neocortical levels in human and animals to determine direct evidence of aberrant synaptic morphology that is associated with deficits in social behaviors. The proposed research advances the study of the pathology of the autism brain through the use of methods that permit the study of single populations of cells within neocortical microcircuitry of highly characterized ASD and control brain donors. The disruption of BDNF-TrkB signaling in the ACC characterized in human autism that is also observed in animal models of ASD will provide a framework for a rational approach to rodent screening of drugs to treat or halt the progression of ASD.